THE BLASTOIDEA 



and so to form a new structure known as the "under lancet-plate" 

 (" sub-lancet "). In this genus the oral surface is raised into more or less 

 of a dome, so that the A are visible in side view. Post. A encloses the anus. 

 Pentremitidea, d'Orbigny (1849, emend. Eth. & Carp., 1882,-86), Dev- 

 onian, Spain, the Eife], and N. America. In the form of the calyx, the 

 complexity of the hydrospires, and other points there is great variability. 

 P. Paillettei, de Vern., however, type of the genus, is not far removed from 

 Phaenoschisma and Cryptoschisma. Eight hydrospire-folds lie on each side 

 of an ambulacrum, entirely covered by the broad lancet-plate. The 

 side-plates are on top of the lancet-plate, so that none of it is visible. 

 The outer side-plates are wedged in between the side-plates and do not 



Fro. VII. 



1, Phac.noschisma Verneuili, food -groove seen from above. lp, lancet-plate ; sp, side-plates, 

 between which are the small outer side-plates. 2, section across part of a radius of the same ; 

 the hydrospires are here in the deltoid, and only a process of the radial is seen (R.pr). 3, part 

 of an ambulacrum of Pentremites, seen from above, x 6 diam. 4, left side of the same further 

 enlarged, showing some of the covering-plates (diagrammatised from Steinmann). 5, section 

 across a radius of Pentremites, x 6 diam. C-8 are diagrams showing the evolution of spiracles : 

 6, in Phaenoschtima ; ends of hydrospire-folds still visible, only an incipient- spiracle. 7, in 

 Orophocrinns ; a long spiracle-slit is enclosed between side-plates and deltoid ridge. 8, in Crypto- 

 schisma; on the left the spiracles are distinctly separated by the deltoid ridge ; on the right 

 the side-platos of adjacent ambulacra meet and the deltoids sink, so that the spiracles appear 

 single, br, brachiole ; br', articular facet for same ; c.p, covering-plates ; A, deltoid crest ; 

 f.g, food-groove ; L, lancet-plate ; 0, peristome ; o.s.p, outer side-plate ; p, pores ; R, radial ; 

 s, spiracle ; s.l, sub-lancet ; s.p, side-plate. 



project beyond their edges. A covered by radial processes, so that only 

 the crests are exposed. The spiracles are clefts on either side the 

 ambulacra, between the deltoid crests and the proximal side-plates. The 

 anus pierces post. A, which therefore has no crest, so that the adjacent 

 spiracles are confluent with the anus ; the opening so formed is called " the 

 anal spiracle." In some species (esp. P. angulata) the side-plates do not 

 cover the lancet-plate any more than they do in some species of Phaeno- 

 schisma. Within the limits of Pentremitidea (as defined by Etheridge & 

 Carpenter) two changes of importance take place. The hydrospires no 

 longer remain extended, with each fold opening into the space below the 



